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Help threading a hole in metal please !

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paulm

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Have decided to make some extra tool shafts for my Rolly Munro hollowing tool, to leave scraper and other attachments fastened to their own shafts for ease of swapping over in use, rather than fiddling around demounting/mounting onto the original shaft.

Have sourced a couple of lengths of the appropriate diameter silver steel and ground a suitable flat on the end of one.

Never having used a metal working tap and die before I decided to have a trial run in a 5mm thick piece of O1 steel using the M8 x 1.25 tap in the screwfix set bought years ago and never used !

Drilled out a hole in stages to 7mm using some cutting paste on the pillar drill, but couldn't get the tap started by hand. Decided to try drilling out to 7.5mm and then tried the tap again, again very difficult to start, did work in the end but a very rough and partial thread in the hole which didn't look complete or even from top to bottom.

Any guidance on correct drill size for an M8 thread and how to ensure it's started off properly, or any hints and tips generally much appreciated before I butcher the carefully ground new tool shaft ?!!!

Cheers, Paul
 

EssexChris

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Hi,
M8 should be a tapping hole diameter of 6.8mm. If your taps are from Screwfix then they might not be of a quality to tap a thread into the silver steel. In a set you normally get a taper, second and bottoming. Do you have a taper tap to start off? For a great range of taps you could try:-

http://www.mscjlindustrial.co.uk

Chris.
 

jasonB

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Are you starting with a "Taper Tap" these have the end ground to a taper so the first wind hardly cuts anything, the next a little more and so on, this helps the tap start and also guides it in the hole.

If you look at this picture the top one is a taper tap.



J
 

Digit

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Everything that's been said above J plus a tip that may help as you are unfamiliar with the task.
Hold the piece of steel in a drill vise, which should be clamped to the drill table. With the hole drilled replace the drill bit with the taper tap and rotate the drill chuck by hand whilst bearing down as though drilling. Once the tap has started to bite continue tapping in the normal manner. This will help you tap the hole correctly.
If your Taps are carbon steel, dump 'em!

Roy.
 

paulm

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Interesting about the taper taps, my set just has the one M8 size and its parallel along it's length, I thought it would be a lot better if tapered ! Will have to take a look for a proper set I think, and some finer increments of drill bit as I only have half mil' sizes currently.

The drill press tip sounds helpful Roy, once I have got the right bit's and pieces together.

Thanks all !

Cheers, Paul
 

Harbo

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Check your drill size too - I've found that a lot of my drills are undersized?

Rod
 

EssexChris

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Another tip is to remember once the thread starts cutting, keep turning the tap back to clear the swarf out. If you just run straight through the thread could strip or the tap could snap.
 

Cheshirechappie

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O1 steel is pretty challenging stuff to thread, and so is silver steel - especially with a coarse thread like M8. I'd use a tapping drill of about 7.2mm, which will give a 60% thread engagement - more than strong enough for most purposes. Start with the taper tap, using lashings of cutting compound, and whatever method you have for keeping the tap square to the job. Go until there is significant resistance, then take out the taper tap, and wind in the second. When this seems to have gone as far as it wants, take it out and go back with the taper. Carry on thus until the hole is tapped. The cutting compound will help a lot in keeping the thread 'clean' and free of tearing.

Don't even try to wind a die down silver steel rod - I'd screwcut in an engineer's lathe, and just finish with the die. If you try to cut an M8 thread on an 8mm bar, especially of something that hard, you'll just break the die, even assuming you could get it to 'start' in the first place. If you could reduce the bar to about 7.75mm o/d, you might stand a chance - but screwcutting first is the only sure way.
 

paulm

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Popped out to the local Cromwell Tools this morning and invested in a set of 6mm and 8mm taps, each taper,second and fine, a decent tap handle and a lifetime supply of cutting paste (why don't they do smaller cans ?!!!).

The trial hole went nice and easily this time so moved onto doing the tool shaft and did this in the drill press (by hand) as suggested to keep the thread nice and vertical. Worked a treat, nice and easy with the right kit :)



I can see a few more tools being made up now that I can tap a decent thread :)

Thanks for the help guys.

Cheers, Paul
 

Harbo

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Nice to see they sell UK made tools.
Where's your nearest Cromwells - I know of one in Soton?

I could have sent you some cutting paste - I bought some from Axy last week? :)

Rod
 

paulm

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Yes, they seem very decent taps Rod as far as I can tell, and not too extortionate at £13 +vat for a set of three. I got the 6mm and 8mm for now and can pick up any other sizes as/when/if needed.

There's a Cromwells about 5 or 10 minutes from me luckily, on the small industrial estate near B&Q, I always tend to forget it's there but dead handy especially on a Saturday morning when it's very quiet and not packed with the trade guys.

Might be able to get stuff cheaper by searching around on the internet, but sometimes it's just easier to go and pick stuff up when you need it, saves the p&p too.

Cheers, Paul
 

Losos

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paulm":2yq1zrmn said:
once I have got the right bit's and pieces together.
Paul, I made the same mistake many years ago and bought a cheap carbon steel tap & die set, it was rubbish :oops:

I was about to bin them when I thought it might be useful just for cleaning threads that are slightly rusted or maybe gunged up with glue etc. and that's what I've used them for ever since. The 'proper hi-speed steel' set (That like you, I had to go out and buy :lol: ) is only used for threading purposes now, still can be tricky on material like stainless steel etc.
 

Paul Hannaby

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I wonder if the time, materials and tools made all this worthwhile when the shaft for the Munro tool works out as an extra £20 if you buy it with the shear scraper?
 

Harbo

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Sadly the smaller they are the more expensive they become - a set of 12BA (which I am looking out for) on offer at £127!!
Ouch!

Rod
 

paulm

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Paul Hannaby":pquhwmss said:
I wonder if the time, materials and tools made all this worthwhile when the shaft for the Munro tool works out as an extra £20 if you buy it with the shear scraper?
Fair point Paul, not cost effective for a one-off, but with care the tools should last for years and facilitate a few other projects too, and I'm always happy to pick up new skills and techniques along the way.

Cheers, Paul
 

Digit

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And in any case, we are idiots, we make things! :lol:

Roy.
 

paulm

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Had some more time in the workshop today and decided to cannibalize a Robert Sorby pick type hollowing tool that I had never got on with for some reason, finding the tool stem too short and the flat shaft limiting as to angle of presentation of the cutter.

So, drilled out the special washer that has the groove machined in it to fit over the profile of the pick, to fit a 6mm cap screw, ground a flat on a second spare piece of 5/8th silver steel, drilled and tapped a 6mm hole to suit, job done (hammer) :D



Bottom of the pic is the donor Sorby tool, above that the new pick tool shaft, and above that the scraper shaft made yesterday and the original Munro cutter head.

Gave the pick tool a quick go on a piece of scrap and worked great, vibration free and solid, the extra mass of the shaft and handle, and the ability to change the angle of presentation of the cutting edge have transformed it for me compared to the original.

Will have to think of some more things to drill and thread now to get value out of the new taps :lol:

Would be great if a mod' could please move this thread to to the woodturning section where it might be more relevant/useful for folk ?

Cheers, Paul
 

Digit

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If you're not careful you'll end up like me! (Shocking thought!)
I make just about everything I can make, the satisfaction of using a tool that you have modified or crafted to suit is very considerable.
My planes are home made, my table saw mitre gauge is home made, the table saw short fence like wise, along with taper jig, tenoning jig, mitre and cross cut sleds, feather boards, router table, special router for locked mitre cutters, overhead router, and the list goes on!
After all, this is what we are about, we make things!

Roy.
 
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